1. Field of the Invention
The present invention relates to a constant current generating circuit for use in a portable phone or the like driven by a battery.
2. Description of the Related Art
Various types of constant current generating circuits have been designed to date. Patent Document 1 listed below, for example, discloses a constant current generating circuit comprised of a reference voltage generating circuit, an operational amplifier, and a current mirror circuit.
[Patent Document 1] Japanese Patent Application Publication No. 6-282338
The constant current generating circuit disclosed in Patent Document 1 adjusts a ratio S (=W/L) of a channel width W to a channel length L with respect to an enhance-type field effect transistor (hereinafter referred to as an FET) and a depletion-type FET provided in the reference voltage generating circuit. Trough this adjustment, the reference voltage is configured to have a negative coefficient such that the reference voltage decreases in response to an increase in ambient temperature. This negative coefficient cancels out a negative coefficient of a resistor made of polysilicon whose voltage decreases in response to an increase in ambient temperature. Hereinafter, a coefficient that specifies a voltage level responsive to an increase/decrease in ambient temperature is referred to as a temperature coefficient. When the voltage decreases in response to an increase in ambient temperature, for example, the temperature coefficient is a negative temperature coefficient. When the voltage increases in response to an increase in ambient temperature, on the other hand, the temperature coefficient is a positive temperature coefficient.
The constant current generating circuit as described above has three drawbacks as described in the following.
(1) An optimum ratio S of an enhance-type or depletion-type FET varies depending on the threshold of the FET. Because of this, it is difficult for a single set of FETs to generate a reference voltage that cancels out a negative temperature coefficient of a resistor.
(2) In order to obviate the above problem, the ratio S may be adjusted by use of laser trimming during a manufacturing step (after the diffusion and generating step). With this method, however, not only the temperature coefficient of the reference voltage but also the potential of the reference voltage is changed. For the purpose of using the circuit as a constant current generating circuit, therefore it becomes necessary to perform trimming also on the resistor made of polysilicon. This results in an undesirable increase in the circuit scale.
(3) The reference voltage generated by this constant voltage generating circuit has a negative temperature coefficient to cancel out the temperature coefficient of a resistor. If there is a need for a reference voltage that has a zero or substantially zero temperature coefficient, thus, it is necessary to provide another reference voltage.
Accordingly, there is a need for a constant current generating circuit that is robust against variation in the manufacturing process and that does not require trimming for adjusting the temperature coefficient to zero after the manufacturing. Also, there is a need for a reference voltage generating circuit that can generate a reference voltage having a zero or substantially zero temperature coefficient.